Anivia's Q possibly has a wrong network relevancy setting
It's the same bug as:
The bug report above is a little bit inaccurate. The Q is spawned on the server, it just doesn't get replicated on the client, which makes it "invisible" to some players.
You have recently added particle effects to Anvia's Q. My best guess is that you have accidentally overwritten the network relevancy setting of her Q to the network relevancy of a particle effect.